The invention relates to water-in-oil emulsion type blasting agents exemplified by Bluhm, U.S. Pat. No. 3,447,978, which have many advantages over conventional slurry blasting compositions, dynamites, ANFO, and aqueous gelled explosives. The emulsion explosive compositions of Bluhm now in common use in the industry have the following components; (a) a discontinuous aqueous phase comprising discrete droplets of an aqueous solution of inorganic oxygen-releasing salts; (b) a continuous water-immiscible organic phase throughout which the droplets are dispersed; (c) an emulsifier which forms an emulsion of the droplets of oxidizer salt solution throughout the continuous organic phase; and (d) a discontinuous gaseous phase.
Water-in-oil emulsion explosive compositions require uniformly dispersed void spaces provided by gas bubbles or a void-providing agent to obtain explosive performance. Therefore, maintaining the uniformly dispersed void spaces in the water-in-oil emulsion explosive is important in achieving good detonation performance and good shelf life. Furthermore, the manner in which void spaces are treated may affect the explosive properties of the emulsion explosive.
Void spaces can be provided by gas bubbles which are mechanically or physically mixed or blown into an emulsion explosive. Voids can also be formed in an emulsion explosive by a chemical gassing agent, or mixed into an emulsion explosive by a void-providing agent, such as hollow microspheres, expanded perlite or styrofoam beads.
A disadvantage of air or gas bubbles results from the fact that they are compressible under high pressures. If subjected to high pressure and compressed, the overall density of the emulsion explosive composition is increased and the composition is no longer detonable and desired explosive performance is reduced. The above phenomenon of density increase and desensitization of an explosive composition is known as precompression or dead pressing. Of course, hollow microspheres of resin or glass can withstand higher pressures than gas or air bubbles, but they too have a critical point of pressure at which they collapse and density reduction takes place.
Emulsion explosive compositions employing hollow microspheres or gas/air bubbles are particularly vulnerable to dead pressing in large blasting applications where holes in a blast pattern are detonated at varying time sequences. An undetonated borehole loaded with an emulsion explosive composition with hollow microspheres can experience dead pressing resulting from a desensitizing shockwave from an adjacent previously fired borehole. The impact of the adjacent charge compresses the undetonated charge, thus increasing its density to the point where it becomes undetonable (i.e., will not detonate reliably using a No. 8 cap).
To overcome the above phenomenon, it has been suggested in U.S. Pat. No. 4,474,628 that one should use stronger hollow microspheres which can withstand greater hydrostatic pressures and thus remain detonable. This suggested solution is both costly and can cause emulsion breakdown problems.